1. Technical Field
This invention relates generally to a voltage clamping circuit and, more particularly, to a circuit for clamping input voltages to CMOS integrated circuits near the voltage rails and preventing forward biasing of semiconductor junctions.
2. Discussion
Clamping devices are commonly employed to limit the voltage input to electrical circuits, and are especially important for maintaining a reliable rail-to-rail voltage input so as to prevent damage to integrated circuitry on a semiconductor chip. Voltage clamping is particularly significant with regard to the CMOS family of digital logic circuits. Without the proper clamping device, the delicate input circuits to CMOS integrated circuitry may be easily destroyed if the input voltage deviates from the rail-to-rail output voltage range.
More specifically, when CMOS integrated circuitry is subjected to positive voltage transients, it is generally necessary to prevent the input voltage from exceeding the upper rail voltage limit. Excessive voltage inputs, which could be caused by static electricity discharges that may occur during handling operations, could damage the electrical circuitry. Similarly, when CMOS integrated circuitry is subjected to negative transients, it is generally desirable to prevent the input voltage from falling below the lower rail voltage limit. Quite often, the lower voltage rail limit is set to ground. Negative transients can potentially cause undesirable effects such as the inducement of excessive current leakage through the associated semiconductor chip. Furthermore, the existence of positive or negative transients outside of the rail-to-rail voltage range can cause undesirable crosstalk between multiplexed inputs.
To handle the positive voltage transient situation, conventional clamping approaches have employed diode clamps. According to this approach, input-protection diodes are typically added to the positive rail input and operate to prevent an input voltage from exceeding a selected upper rail voltage limit. For negative voltage transients, conventional approaches commonly rely on the forward biasing of a parasitic input junction to the CMOS substrate to suppress adverse effects caused by the input voltage falling below the lower rail voltage.
According to the conventional approaches, positive and/or negative transient voltages can be clamped by allowing junctions to forward bias and, in the negative transient, cause minority carrier injection into the semiconductor substrate. A CMOS chip can go into a "latchup" state if the input voltage is driven beyond the supply voltage. The resulting current through the clamp diodes generally turns "on" a pair of parasitic cross-connected transistors that are a side effect of the junction isolated CMOS process. Furthermore, with the diode clamp, the input voltage may drop below ground through the inherency of the parasitic diode. That is, if the input voltage goes below ground, the parasitic diode turns "on" and causes an undesirable reverse current which can result in undesirable current leakage through associated CMOS devices on the semiconductor chip.
Prior art approaches, such as the above parasitic diode approach, generally tend to suffer for a number of reasons. For example, the presence of undesirable reverse currents can affect circuit accuracy and possibly lead to circuit damage. In addition, the conventional parasitic approach generally operates with a dependency on process and temperature conditions. As a consequence, changing conditions may effect the accuracy of the input voltages and thus diminish the reliability of the CMOS integrated circuits over varying conditions.
It is therefore desirable to provide for a circuit for clamping input voltages to CMOS devices which prevents forward biased junctions.
It is also desirable to provide for a voltage clamping circuit for CMOS integrated circuits which prevents undesirable current leakage and minority carrier injection into the substrate.
It is further desirable to provide for a voltage clamping circuit which may clamp CMOS IC input voltages near the rails independent of process and temperature conditions.